Non-aqueous pigment ink

ABSTRACT

A non-aqueous pigment ink includes a pigment, a non-aqueous solvent, a non-water-soluble resin and a water-soluble resin, wherein the non-water-soluble resin is an acrylic polymer formed by a copolymer of a monomer mixture containing at least an alkyl (meth)acrylate (A) having a C8 to C18 alkyl group and a monomer (B) having a β-diketone group or β-keto acid ester group.

The present application is a divisional of U.S. application Ser. No.13/933,973, filed Jul. 2, 2013, which claims priority to JapaneseApplication No. 2012-151614 filed Jul. 5, 2012. The disclosure of U.S.application Ser. No. 13/933,973 is incorporated by reference herein intheir entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-aqueous pigment ink that issuitable for use with an inkjet recording device, and in particular to anon-aqueous pigment ink that is capable of contributing to power saving,and capable of reducing or eliminating print-through (striking-through),thereby achieving high print density.

2. Description of the Related Art

An inkjet recording system ejects a highly fluid inkjet ink as an inkparticle from a very thin head nozzle to record an image on a recordingmedium, which is positioned to face the nozzle. Because of low noise andability of high-speed printing, the inkjet recording systems are rapidlybecoming widely used in recent years. As inks for use with the inkjetrecording systems, so-called non-aqueous pigment inks, which are formedby finely dispersing a pigment in a non-water-soluble solvent, areknown.

In recent years, in view of resource environment and energy saving, itis desired to reduce the power consumption of devices, such as printers,as low as possible, and there are ever increasing demands forpower-efficient deices for power saving in the field of inkjet printing.

In inkjet recording devices, ink in an ink chamber provided in an inkjethead is ejected from a nozzle when a pressure is applied to the inkchamber. The ink ejected from the nozzle flies with leaving a tailbehind it, and there is a time difference and a velocity differencebetween the leading part and the trailing part of the flying ink. In alow temperature environment, viscosity of the ink increases, andtherefore it is necessary to apply a higher driving voltage to theinkjet head for ejecting a desired amount of ink, resulting in increasedpower consumption. Further, the ink ejection with the higher drivingvoltage tends to form satellites. The satellites are deposited on arecording medium and degrade the print quality. In order to ensure theprint quality, conventionally, in a low temperature environment wherethe satellites are likely to be formed, a recording operation is startedafter a so-called warming-up operation to warm the inkjet head isperformed.

That is, in a low temperature environment where the satellites arelikely to be formed, the recording operation is started after thewarming-up operation, and therefore electric power for the warming-upoperation is also consumed. Further, the longer the time required forthe warming-up operation, the longer the time taken for recording animage. This is a problem not only in view of the electric power but alsoin view of the operation time of the user. To address this problem fromthe ink side, it is effective for power saving to reduce the inkviscosity in a low temperature environment. To this end, it is veryeffective to provide an ink with lower viscosity. The ink viscosity canbe reduced by reducing the amount of colorant, or the amount of powder,in the ink. However, this results in lower print density, and thus lowerimage quality.

For example, an ink using a colorant which is formed by combining apigment, a dispersant, and a water-soluble resin including two or moreprimary and/or secondary amino groups in a molecule, which react withreactive functional groups of the dispersant and are chemically-bound tothe dispersant, has been proposed in U.S. Pat. No. 7,767,013(hereinafter, Patent Document 1). This ink has high storage stability,high pigment dispersibility, and high ejection stability with noclogging in the nozzle. A dispersant, in general, stabilizes an ink byrepeating adsorption and desorption onto and from the surface of apigment to achieve an equilibrium state. With the colorant disclosed inPatent Document 1, however, it is necessary to increase the amount ofthe dispersant to stabilize the ink in a low temperature environment,and it is difficult to reduce the viscosity to a level where thesatellites are reduced or eliminated.

Therefore, with the ink of Patent Document 1, it is necessary to warmthe ink to reduce the ink viscosity, and there still is the problem ofincreased power consumption. In particular, in the case where the ink isused with a circulation-type inkjet recording device, the volume of inkto be warmed is larger. This results in higher electric powerconsumption and longer time taken for warming, and thus longer timetaken for outputting the first print. On the other hand, in order toreduce the viscosity, it is necessary to reduce the amount of thedispersant. In this case, however, it is difficult to ensure the pigmentdispersion stability.

Use of a hydrocarbon-based non-polar solvent with high boiling point andlow viscosity (which will hereinafter be simply referred to as“hydrocarbon-based non-polar solvent”) can provide an ink with lowviscosity. When the hydrocarbon-based non-polar solvent is used as anink solvent, the polarity of the ink solvent is changed, and this mayresult in poorer pigment dispersion stability. However, it is believedthat this problem can be solved by changing the structure of thedispersant. The applicant has proposed, in U. S. Patent ApplicationPublication No. 2011/0046298 (hereinafter, Patent Document 2), anon-aqueous pigment ink containing non-water-soluble resin dispersingparticulates, which are capable of dispersing a pigment.

SUMMARY OF THE INVENTION

However, when the non-water-soluble resin dispersing particulatesdisclosed in Patent Document 2 are dispersed in a hydrocarbon-basednon-polar solvent, functional groups (urethane groups) to be adsorbedonto the pigment are oriented inward and alkyl groups having highaffinity to the hydrocarbon-based non-polar solvent are orientedoutward, and the non-water-soluble resin dispersing particulates are noteasily adsorbed onto the pigment. Therefore, the pigment dispersibilitycannot be ensured when the amount of the non-water-soluble resindispersing particulates is small. It is therefore necessary to prescribea sufficiently large amount of the non-water-soluble resin dispersingparticulates relative to the pigment, and this results in higher inkviscosity. On the other hand, to ensure the pigment dispersibility, thehydrocarbon-based non-polar solvent needs to have good affinity to thepigment. However, if the affinity is excessively high, thehydrocarbon-based non-polar solvent tends to drag the pigment into arecording medium when it penetrates into the recording medium. Thisresults in lower print density and higher tendency of print-through.

That is, in order to provide an ink with low viscosity, it is better touse a smaller amount of the dispersant. To this end, it is necessarythat the dispersant is dissolved in the non-aqueous solvent, rather thanin the particle form where the groups to be adsorbed onto the pigmentare oriented inward. Further, as described above, while the pigmentdispersant repeats adsorption and desorption onto and from the surfaceof the pigment to achieve the equilibrium state to stabilize the inksystem, stabilization of the ink system can be achieved with a smalleramount of pigment dispersant if the pigment dispersant can beimmobilized onto the surface of the pigment.

Further, while the water-soluble resin disclosed in Patent Document 1can stabilize the ink system, it has been found that an ink obtained bymixing the water-soluble resin in the non-water-soluble resin dispersingparticulates disclosed in Patent Document 2 has high wetting property toa nozzle plate used in an inkjet head, and this may cause the inkejected from the nozzle to be deviated, or the ink to be not ejectedfrom the nozzle. Further, the ink adhering to the nozzle plate may betransferred onto a printing paper sheet, or the like, to contaminate theprint.

In view of the above-described circumstances, the present invention isdirected to providing a non-aqueous pigment ink which achieves low inkviscosity to contribute to power saving, has good storage stability(pigment dispersion stability) and can reduce or eliminate theprint-through at the same time, thereby achieving high print density.

An aspect of the non-aqueous pigment ink of the invention is anon-aqueous pigment ink including a pigment, a non-aqueous solvent, anon-water-soluble resin and a water-soluble resin, wherein thenon-water-soluble resin is an acrylic polymer formed by a copolymer of amonomer mixture containing at least an alkyl (meth)acrylate (A) having aC8 to C18 alkyl group and a monomer (B) having a β-diketone group orβ-keto acid ester group.

The non-water-soluble resin herein refers to a resin with a solubilityin water of 23° C. is 0.5 mass % or less at 20° C.

It is preferable that the acrylic polymer has a comb-shaped structurehaving urethane groups as side chains to the main chain of the acrylicpolymer.

It is preferable that the water-soluble resin is a polyethylene iminehaving a mass average molecular weight in the range from 200 to 2000, ora modified polyethylene imine obtained through addition reaction betweenthe polyethylene imine and one of an acrylate or a vinyl compound, wherea ratio of the acrylate or the vinyl compound to the polyethylene imineis not less than 0.3 molar equivalent and less than 1 molar equivalentto the total amine number of the polyethylene imine of 1 molarequivalent.

It is preferable that the content of the acrylic polymer is in the rangefrom 0.1 to 1.0 in mass ratio relative to the pigment.

It is preferable that the content of the acrylic polymer is in the rangefrom 0.1 to 20 in mass ratio relative to the water-soluble resin.

It is preferable that the content of the water-soluble resin is in therange from 0.01 to 0.5 in mass ratio relative to the pigment.

The non-aqueous pigment ink of the invention is a non-aqueous pigmentink including a pigment, a non-aqueous solvent, a non-water-solubleresin and a water-soluble resin, where the non-water-soluble resin is anacrylic polymer formed by a copolymer of a monomer mixture containing atleast an alkyl (meth)acrylate (A) having a C8 to C18 alkyl group and amonomer (B) having a β-diketone group or β-keto acid ester group.Therefore, an ink with low viscosity can be provided, which allowsensuring the low temperature suitability and the pigment dispersionstability and reducing or eliminating the print-through at the sametime, thereby achieving high print density.

Therefore, the non-aqueous pigment ink of the invention is preferablyusable as an inkjet ink. Further, the non-aqueous pigment ink of theinvention has low ink viscosity even in a low temperature environment,and is preferably usable, in particular, with circulation-type inkjetrecording devices, which take longer time and more electric power forwarming-up.

In the case where the water-soluble resin is a modified polyethyleneimine obtained through an addition reaction between the polyethyleneimine and one of an acrylate and a vinyl compound, where the ratio ofthe acrylate or the vinyl compound to the polyethylene imine is not lessthan 0.3 molar equivalent and less than 1 molar equivalent to the totalamine number of the polyethylene imine of 1 molar equivalent, asufficiently high effect of improving print density is provided,although the effect is slightly smaller than that of the case where thepolyethylene imine is used, and an effect of increasing ink repellencyfrom the nozzle plate is further provided, since the active hydrogen of—N—H of the amino group is alkylated (—N—R) to reduce affinity to thenozzle plate. Therefore, such a situation that the ink ejected from thenozzle is deviated or the ink is not ejected from the nozzle is reducedeven without frequent wiping, and it is less likely that the inkadhering to the nozzle plate is transferred onto a printing paper sheet,or the like, to contaminate the print.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A non-aqueous pigment ink (which may hereinafter simply be referred toas “ink”) of the invention contains a pigment, a non-aqueous solvent, anon-water-soluble resin and a water-soluble resin.

The non-water-soluble resin is an acrylic polymer formed by a copolymerof a monomer mixture containing at least an alkyl (meth)acrylate (A)having a C8 to C18 alkyl group and a monomer (B) having a β-diketonegroup or β-keto acid ester group.

The C8 to C18 alkyl group of the functional group of (A) is highlycompatible with a hydrocarbon-based non-polar solvent of the non-aqueoussolvent, which will be described later, thereby providing the dissolvedstate in the non-aqueous solvent. The β-diketone group or β-keto acidester group, which is the functional group of (B), serves to reduce theink viscosity to improve the low temperature suitability. Further,reducing the increase of viscosity contributes to electrostaticaggregation and fixing of the ink when the ink lands on a recordingmedium, and the improvement of print density and the reduction orelimination of print-through are achieved as a result.

If the carbon number of the alkyl group is 19 or more, thenon-water-soluble resin tends to be solidified at low temperature andthe low temperature suitability is impaired. On the other hand, if thecarbon number of the alkyl group is 7 or less, the compatibility withthe hydrocarbon-based non-polar solvent decreases and the stable pigmentdispersion cannot be achieved, resulting in poorer storage stability andhigher ink viscosity. In a low temperature environment, the inkviscosity becomes even higher, and the low temperature suitability isimpaired. It is more desirable that the alkyl group is a C12 to C18alkyl group.

The C8 to C18 alkyl group forming the functional group may be linear orbranched. Specific examples thereof include octyl group, nonyl group,decyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecylgroup, hexadecyl group, heptadecyl group and octadecyl group, and two ormore species thereof may be included.

With respect to the β-diketone group or β-keto acid ester group formingthe functional group, preferred examples of the β-diketone group includeacetoacetyl group and propion acetyl group, and preferred examples ofthe β-keto acid ester group include acetoacetoxy group and propionacetoxy group.

The molecular weight (mass average molecular weight) of the acrylicpolymer is not particularly limited. However, if the ink of theinvention is used as an inkjet ink, the molecular weight is preferablyin the range from about 5,000 to about 50,000, and more preferably inthe range from about 10,000 to about 30,000 in view of ejectability ofthe ink.

The glass-transition temperature (Tg) of the acrylic polymer ispreferably not higher than room temperature, and more preferably 0° C.or less. With the glass-transition temperature in this range, filmformation can be promoted at the room temperature when the ink is fixedon a recording medium.

The alkyl (meth)acrylate (A) is an alkyl (meth)acrylate including a C8to C18 alkyl group, and forms the main chain of the acrylic polymertogether with the monomer (B), where the alkyl group forms a functionalgroup of the main chain. Preferred examples of the alkyl (meth)acrylate(A) include palmityl/stearyl methacrylate (C16/C18), cetyl acrylate(C16), dodecyl methacrylate (C12), dodecyl acrylate (C12), 2-ethylhexylmethacrylate (C8) and 2-ethylhexyl acrylate (C8), which may be usedsingly or in an appropriate combination.

The monomer (B) is a (meth)acrylate or (meth)acrylamide including theβ-diketone group or β-keto acid ester group, and forms the main chain ofthe acrylic polymer together with the alkyl (meth)acrylate (A), wherethe β-diketone group or β-keto acid ester group forms a functional groupof the main chain. The monomer (B) serves to reduce the ink viscosity tofurther improve the low temperature suitability. Further, reducing theincrease of viscosity contributes to electrostatic aggregation andfixing of the ink when the ink lands on a recording medium, and theimprovement of print density and the reduction or elimination ofprint-through are achieved as a result.

Preferred examples of the monomer (B) include a (meth)acrylate and a(meth)acrylamide which include the β-diketone group or β-keto acid estergroup in the ester chain. More specific examples of the monomer (B)include: acetoacetoxy alkyl (meth)acrylates, such as acetoacetoxy ethyl(meth)acrylate; and acetoacetoxy alkyl (meth)acrylamides, such ashexadione (meth)acrylate and acetoacetoxy ethyl (meth)acrylamide, whichmay be used singly or in combination of two or more species.

The content of the alkyl (meth)acrylate (A) in the above-describedmonomer mixture (the alkyl (meth)acrylate (A) and the monomer (B)) ispreferably 30 mass % or more, more preferably in the range from 40 to 95mass %, and even more preferably in the range from 50 to 90 mass %. Thecontent of the monomer (B) in the above-described monomer mixture ispreferably in the range from 3 to 30 mass %, and more preferably in therange from 5 to 20 mass %.

Copolymerization of the above-described monomers can easily be achievedby known radical copolymerization. The reaction system may preferably besolution polymerization or dispersion polymerization. In this case, inorder to achieve the molecular weight of the acrylic polymer within theabove-described preferred range after the polymerization, it iseffective to use a chain transfer agent in combination during thepolymerization. Examples of the chain transfer agent include thiols,such as n-butyl mercaptan, lauryl mercaptan, stearyl mercaptan andcyclohexyl mercaptan.

As a polymerization initiator, a known thermal polymerization initiator,such as an azo compound (such as AIBN (azobisisobutyronitrile)) or aperoxidize (such as t-butyl peroxybenzoate, t-butylperoxy-2-ethylhexanoate (PERBUTYL O, available from NOF Corporation))may be used. As another example, a photopolymerization initiator, whichgenerates radicals when exposed to an active energy ray, may be used. Asa polymerization solvent used for the solution polymerization, apetroleum solvent (aroma-free (AF)), etc., may be used. As thepolymerization solvent, it is preferable to select one or more ofsolvents which are usable as the non-aqueous solvent in the ink (whichwill be described later). For the polymerization reaction, other agentsusually used in polymerization, such as a polymerization inhibitor, apolymerization promoter, a dispersant, etc., may be added to thereaction system.

The acrylic polymer in the invention preferably has a comb-shapedstructure including urethane groups as side chains to the main chain ofthe acrylic polymer formed by the alkyl (meth)acrylate (A) and themonomer (B). The C8 to C18 alkyl group of the alkyl (meth)acrylate (A)serves to improve affinity to the hydrocarbon-based non-polar solvent ofthe non-aqueous solvent, which will be described later, to ensure thesolubility in the solvent. On the other hand, the side chains formed byurethane groups serve to adsorb onto the pigment to improve the storagestability.

Introduction of the urethane groups of the side chains can be achievedby using a (meth)acrylate having functional groups capable of reactingwith amino groups in addition to the alkyl (meth)acrylate (A) and themonomer (B), i.e., through a reaction among the functional groupscapable of reacting with amino groups, an amino alcohol and a polyvalentisocyanate compound, which will be described later. Preferred examplesof the functional groups capable of reacting with amino groups include aglycidyl group, a vinyl group and a (meth)acryloyl group.

An example of the (meth)acrylate including a glycidyl group is glycidyl(meth)acrylate, and preferred examples of the (meth)acrylate including avinyl group include vinyl (meth)acrylate and 2-(2-vinyloxyethoxy)ethyl(meth)acrylate. Examples of the (meth)acrylate including a(meth)acryloyl group include dipropylene glycol di(meth)acrylate and1,6-hexanediol di(meth)acrylate. The (meth)acrylate may include two ormore species. It should be noted that the (meth)acrylate havingfunctional groups capable of reacting with amino groups may be also usedin a case where the urethane groups are not introduced.

As the amino alcohol reacts with and binds to the functional groupscapable of reacting with amino groups, and an addition reaction betweena hydroxy group of the amino alcohol and an isocyanate group (R¹N═C═O)of the polyvalent isocyanate compound occurs as shown below, theurethane groups (urethane bonds) (carbamates: R¹NHCOOR) are introduced.

R¹N═C═O+R—OH→ROCONHR¹

where R— represents an amino alcohol moiety bound to the functionalgroup of the copolymer.

In this manner, the urethane groups serving as pigment adsorbing groupsare introduced.

Examples of the amino alcohol include monomethyl ethanolamine,diethanolamine and diisopropanolamine. Among them, dialkanolamine(secondary alkanolamine) represented by the general formula: (HOR)₂NH(where R is a divalent hydrocarbon group), which has two hydroxy groups,is preferable since the number of formed urethane groups is increased.These amino alcohols may be used in combination of two or more species.

In the case where the urethane groups are introduced, the amount of theamino alcohol to be reacted is preferably 0.05 to 1 molar equivalent,and more preferably 0.1 to 1 molar equivalent relative to the functionalgroups capable of reacting with amino groups, of the (meth)acrylate, inview of the introduction of the urethane groups. When the amount of theamino alcohol is less than 1 molar equivalent, unreacted functionalgroups of the (meth)acrylate having functional groups capable ofreacting with amino groups are left. However, it is believed that theunreacted functional groups serve as pigment adsorbing groups.

Examples of the polyvalent isocyanate compound include aliphatic,alicyclic and aromatic compounds, such as 1,6-diisocyanate hexane,1,3-bis(isocyanatomethyl)benzene, 1,3-bis(isocyanatomethyl)cyclohexaneand 1,5-naphthalene diisocyanate, which may be used in combination oftwo or more species. The amount of the polyvalent isocyanate compound tobe reacted is preferably almost equivalent (0.98 to 1.02 molarequivalent) to hydroxy groups included in prepared raw materials, sothat no unreacted raw materials are left when the urethane groups areintroduced through the reaction with the hydroxy groups.

The mass ratio between the copolymer moieties and the introducedurethane group moieties of the acrylic polymer is preferably in therange from 80:20 to 99:1, and more preferably in the range from 85:15 to95:5. The mass of the copolymer moieties of the acrylic polymer refersto the total mass of the monomers used in the copolymerization, and themass of the introduced urethane group moieties refers to the total massof the amino alcohol and the polyvalent isocyanate compound used in thereaction. The urethane group moieties have high pigment adsorptioncapacity. However, although one might expect that a higher mass ratio ofthe urethane group moieties results in a higher pigment adsorption rate,a mass ratio of the urethane group moieties higher than 20 results inpoorer compatibility with the solvent and the amount of freenon-water-soluble resin is increased, resulting in lower pigmentadsorption rate.

The content of the acrylic polymer relative to the total amount of theink is preferably 0.1 mass % or more, and more preferably 1 mass % ormore, in view of ensuring the pigment dispersibility. If the content ofthe acrylic polymer is excessively high, the ink viscosity is increased,and storage stability in a high temperature environment may be impaired.Therefore, the content of the acrylic polymer is preferably not morethan 20 mass %, and more preferably not more than 10 mass %. That is,the content of the acrylic polymer relative to the total amount of theink is preferably in the range from 1 to 10 mass %, and more preferablyin the range from 2 to 8 mass %.

The content of the acrylic polymer relative to the pigment is preferablyin the range from 0.1 to 1.0 in mass ratio relative to the pigment inview of ensuring the storage stability. If the content of the acrylicpolymer relative to the pigment is excessively low, i.e., less than 0.1in mass ratio, or excessively high, i.e., more than 1.0 in mass ratio,it is difficult to ensure the storage stability.

The content of the acrylic polymer relative to the water-soluble resinis preferably in the range from 0.1 to 20 in mass ratio, and morepreferably in the range from 0.4 to 10 in mass ratio. If the content ofthe acrylic polymer relative to the water-soluble resin is excessivelylow, i.e., less than 0.1 in mass ratio, or excessively high, i.e., morethan 20 in mass ratio, it is difficult to ensure the storage stability.

The mass of the resin (the total amount of the acrylic polymer and thewater-soluble resin) relative to the mass of the pigment is preferably0.2 or more relative to the mass of pigment of 1, in view of ensuringthe pigment dispersing effect, and preferably not more than 1.5 in viewof improving the ink viscosity and avoiding defective ejection due totemporal change.

The content of the water-soluble resin is preferably in the range from0.01 to 0.5, more preferably in the range from 0.05 to 0.3, and mostpreferably in the range from 0.1 to 0.2 in mass ratio relative to thepigment.

The content of the water-soluble resin relative to the total amount ofthe ink is preferably in the range from about 0.1 to about 5 mass %, andmore preferably in the range from 0.5 to 1.5 mass %.

Examples of the water-soluble resin include basic polymericelectrolytes, such as polyethyleneimine (PEI), polyvinylamine andpolyvinylpyridine, and derivatives thereof. In particular, apolyethylene imine having a mass average molecular weight in the rangefrom 200 to 2,000, or a modified polyethylene imine obtained throughaddition reaction between a polyethylene imine having a mass averagemolecular weight in the range from 200 to 2,000 and one of an acrylateor a vinyl compound is preferably usable. The modified polyethyleneimine is preferably one where the ratio of the acrylate or the vinylcompound is not less than 0.3 molar equivalent and less than 1 molarequivalent relative to the total amine number of the polyethylene imineof 1 molar equivalent (which may hereinafter be simply referred to as“modified polyethylene imine”). The amine number here is calculated byfinding an amine number (KOH mg/g) according to “(2) Indicator titrationmethod” of JIS K-7237-1995 (Testing method for total amine numbers ofamine-based hardeners of epoxy resins) and converting the found aminenumber using the molecular weight of 56.11 mg/mmol of KOH.

If the mass average molecular weight of the polyethylene imine is lessthan 200, effect of obtaining high density on plain paper is low. On theother hand, if the mass average molecular weight of the polyethyleneimine is 2000 or more, the storage stability may be impaired dependingon the storage environment. The mass average molecular weight of thepolyethylene imine is more preferably in the range from 300 to 1800, inview of obtaining high density, and a pour point of −5° C. or less toensure good storage stability in a low temperature environment.

As the polyethylene imine, a commercially available polyethylene iminemay be used, and preferred examples thereof include: SP-006, SP-012,SP-018 and SP-200 available from Nippon Shokubai Co., Ltd.; and LupasolFG, Lupasol G20 Waterfree, Lupasol PR8515 available from BASF.

In the modified polyethylene imine, the acrylate and the vinyl compoundmay be used in combination. In this case, the ratio of the acrylate andthe vinyl compound is not less than 0.3 molar equivalent relative to theamine number the polyethylene imine of 1 molar equivalent. If the ratioof the acrylate and the vinyl compound is less than 0.3 molarequivalent, the effect of improving ink repellency from the nozzle plateis weakened.

The mechanism of improvement of the ink repellency from the nozzle plateby using the modified polyethylene imine is estimated as follows. Iminogroups or amino groups (—NH, —NH₂) present in an ink using polyethyleneimine tend to adhere to the nozzle plate of the inkjet head. By usingthe modified polyethylene imine obtained by adding the acrylate or vinylcompound to the imino group or amino group of the polyethylene iminethrough the Michael addition reaction, the ink repellency is increasedand the wetting property to the nozzle plate is improved.

Preferred examples of the acrylate include methyl acrylate, ethylacrylate, t-butyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate.

Preferred examples of the vinyl compound include acrylonitrile, vinylhalides, such as vinyl chloride and vinyl fluoride, and vinyl acetate.

The modified polyethylene imine can be obtained by adding the acrylateor vinyl compound to the imino group or amino group of the polyethyleneimine through Michael addition. Specifically, the modified polyethyleneimine can be prepared by dripping the polyethylene imine and theacrylate or the vinyl compound in diethanolamine heated to 50° C. to 60°C. with stirring, and then maintaining the temperature of 50° C. to 60°C. for 1 to 3 hours.

Even when the acrylic polymer does not have a functional group with highpigment adsorption capacity, such as the urethane group, strong pigmentadsorption capacity can be provided when the acrylic polymer having theβ-diketone group or β-keto acid ester group is used in combination withthe water-soluble resin. It is estimated that interaction among theβ-diketone group or β-keto acid ester group of the non-water-solubleresin, the polar group of the water-soluble resin and the pigmentstabilizes the pigment dispersion. The interaction among the β-diketonegroup or β-keto acid ester group, the polar group of the water-solubleresin and the pigment reduces desorption of the non-water-soluble resinfrom the pigment, thereby allowing further reduction of the amount ofthe non-water-soluble resin to be used. As a result, the low inkviscosity is achieved while ensuring the pigment dispersion stability,thereby achieving more excellent low temperature suitability.

The acrylic polymer having the comb-shaped structure including urethanegroups as side chains has high pigment adsorption capacity due to theurethane groups. However, if the mass ratio of the urethane groups isexcessively high, compatibility with the solvent is impaired and thepigment adsorption rate decreases, resulting in increase of the amountof free non-water-soluble resin, and thus increase of the ink viscosity.However, in the case where the acrylic polymer having the comb-shapedstructure including urethane groups as side chains further has theβ-diketone group or β-keto acid ester group, interaction with thepigment can be enhanced by using the water-soluble resin in combination,and the amount of free non-water-soluble resin can be reduced to achievethe low ink viscosity.

Further, while the affinity of the non-aqueous solvent to thehydrocarbon-based non-polar solvent is improved by the C8 to C18 alkylgroup of the acrylic polymer to ensure the solubility in the solvent, ifthe affinity of the non-aqueous solvent to the pigment is excessivelyhigh, the non-aqueous solvent tends to drag the pigment into a recordingmedium when it penetrates into the recording medium. By combining theacrylic polymer with the water-soluble resin, the pigment dispersionstability is ensured even when the amount of the acrylic polymer isreduced. This allows reducing the amount of the acrylic polymer, therebyreducing the penetration of the pigment. As a result, the print-throughis reduced or eliminated, and the high print density is achieved.

As the non-aqueous solvent, a hydrocarbon-based non-polar solvent orpolar solvent may be used, which may be used singly or in an appropriatecombination of two or more species. In view of providing the lowviscosity, it is preferable to use a hydrocarbon-based non-polarsolvent.

The content of the hydrocarbon-based non-polar solvent is preferably 20mass % or more, more preferably 50 mass % or more, and even morepreferably 80 mass % or more relative to the total mass of the inksolvent. If the content of the hydrocarbon-based non-polar solvent isless than 50 mass % relative to the total amount of the solvent, it maybe difficult to provide sufficiently low viscosity of the ink dependingon the environment in which the ink is used.

When the content of the hydrocarbon-based non-polar solvent is 50 mass %or more relative to the total amount of the ink solvent, even lower inkviscosity and further improvement of the storage stability are achieved.With the content of the hydrocarbon-based non-polar solvent being 50mass % or more relative to the total amount of the ink solvent, almostno free water-soluble resin and non-water-soluble resin are present inthe ink solvent, and the resins gather in the vicinity of the pigmentand are strongly adsorbed onto the surface of the pigment. Therefore, itis estimated that the effect of reducing the ink viscosity is providednot only by the reduction of the viscosity of the solvent itself butalso by the reduction of the amount of free resins in the solvent, andfurther improvement of the pigment dispersion stability can be achieved.

Preferred examples of the hydrocarbon-based non-polar solvent include analiphatic hydrocarbon solvent, an alicyclic hydrocarbon-based solventand an aromatic hydrocarbon solvent. Preferred examples of the aliphatichydrocarbon solvent and the alicyclic hydrocarbon-based solvent include:TECLEAN N-16, TECLEAN N-20, TECLEAN N-22, NISSEKI NAPHTESOL L, NISSEKINAPHTESOL M, NISSEKI NAPHTESOL H, NO. 0 SOLVENT L, NO. 0 SOLVENT M, NO.0 SOLVENT H, NISSEKI ISOSOL 300, NISSEKI ISOSOL 400, AF-4, AF-5, AF-6and AF-7 available from JX Nippon Oil & Energy Corporation; and IsoparG, Isopar H, Isopar L, Isopar M, Exxsol D40, Exxsol D80, Exxsol D100,Exxsol D130, and Exxsol D140 available from Exxon. Preferred examples ofthe aromatic hydrocarbon solvent include NISSEKI CLEANSOL G (alkylbenzene) available from JX Nippon Oil & Energy Corporation, and SOLVESSO200 available from Exxon.

As the above-described polar solvent, polar solvents such as an estersolvent, an alcohol solvent, a higher fatty acid solvent, or an ethersolvent may be used. More specifically, an ester solvent with a carbonnumber in a molecule of 14 or more, such as methyl laurate, isopropyllaurate, hexyl laurate, isopropyl myristate, isopropyl palmitate,isostearyl palmitate, methyl oleate, ethyl oleate, isopropyl oleate,butyl oleate, methyl linoleate, isobutyl linoleate, ethyl linoleate,isopropyl isostearate, soybean oil methyl ester, soybean oil isobutylester, tall oil methyl ester, tall oil isobutyl ester, diisopropyladipate, diisopropyl sebacate, diethyl sebacate, propylene glycolmonocaprate, trimethylol propane tri-2-ethylhexanoate, glyceryltri-2-ethylhexanoate, etc.; an alcohol solvent with a carbon number in amolecule of 12 or more, such as isomyristyl alcohol, isopalmitylalcohol, isostearyl alcohol, oleyl alcohol, etc.; a higher fatty acidsolvent, such as isononanoic acid, isomyristic acid, hexadecane acid,isopalmitic acid, oleic acid, isostearic acid, etc.; or an ethersolvent, such as diethylglycol monobutylether, ethylene glycolmonobutylether, propylene glycol monobutylether, propylene glycoldibutylether, etc., may preferably used. These non-aqueous solvents maybe used singly or in combination of two or more species.

Examples of the pigment include: carbon blacks, such as Furnace Black,Lamp Black, Acetylene Black, Channel Black, etc.; metals or metaloxides, such as copper, iron, titanium oxide, etc.; and organicpigments, such as Ortho Nitro Aniline Black, etc. These pigments may beused singly or in an appropriate combination. Examples of the pigmentsfor color inks include Watching Red, Toluidine Red, Permanent CarmineFB, Disazo Orange PMP, Lake Red C, Brilliant Carmine 6B, QuinacridoneRed, Dioxane Violet, Orthonitro Aniline Orange, Dinitro Aniline Orange,Vulcan Orange, Toluidine Red, Chlorinated Para Red, Brilliant FastScarlet, Naphthol Red 23, Pyrazolone Red, Barium Red 2B, Calcium Red 2B,Strontium Red 2B, Manganese Red 2B, Barium Lithol Red, Pigment Scarlet3B Lake, Lake Bordeaux 10B, Anthocin 3B Lake, Anthocin 5B Lake,Rhodamine 6G Lake, Eosin Lake, Iron Oxide Red, Naphthol Red FGR,Rhodamine B Lake, Methyl Violet Lake, Dioxazine Violet, Naphthol CarmineFB, Naphthol Red M, Fast Yellow AAA, Fast Yellow 10G, Disazo YellowAAMX, Disazo Yellow AAOT, Disazo Yellow AAOA, Disazo Yellow HR,Isoindoline Yellow, Fast Yellow G, Disazo Yellow AAA, PhthalocyanineBlue, Victoria Pure Blue, Basic Blue 5B Lake, Basic Blue 6G Lake, FastSky Blue, Alkali Blue R Toner, Peacock Blue Lake, Iron Blue,Ultramarine, Reflex Blue 2G, Reflex Blue R, Alkali Blue G Toner,Brilliant Green Lake, Diamond Green Thioflavin Lake, PhthalocyanineGreen G, Green Gold, Phthalocyanine Green Y, purple oxide, zinc oxide,titanium oxide, calcium carbonate, clay, barium sulfate, alumina white,aluminum powder, bronze powder, daylight fluorescent pigments, pearlpigments, etc. These pigments may be used singly or in an appropriatecombination.

The content of the pigment in the ink is usually in the range from 0.01to 20 mass %. In view of the print density and the ink viscosity, thecontent of the pigment in the ink is preferably in the range from 1 to15 mass %, and more preferably in the range from 5 to 10 mass %.

Besides the above-described components, the ink of the invention mayinclude conventional additives. Examples of the additives may include asurfactant, such as an anionic, cationic, amphoteric or nonionicsurfactant, an antioxidant, such as dibutylhydroxytoluene, propylgallate, tocopherol, butylhydroxyanisol or nordihydroguaiaretic acid,etc.

For an ink for use with an inkjet recording system, a suitable range ofthe ink viscosity varies depending on the nozzle diameter of theejection head, the ejection environment, etc; however, in general, it ispreferably in the range from 5 to 30 mPa·s at 23° C., more preferably inthe range from 5 to 15 mPa·s, and an ink viscosity of about 10 mPa·s at23° C. is suitable for use with an inkjet recording device. The inkviscosity herein refers to a value of the ink viscosity under a shearstress of 10 Pa when the shear stress is increased from 0 Pa at a rateof 0.1 Pa/s at 23° C.

The ink of the invention can be prepared by mixing the pigment, thenon-water-soluble resin, the non-aqueous solvent and the water-solubleresin, dispersing the pigment using a dispersing means, such as a ballmill or a bead mill, and filtering the mixture, as desired, using aknown filter, such as a membrane filter. It should be noted that, in thecase where a polyethylene imine is used as the water-soluble resin,which is often slightly soluble or hardly-soluble in commonly-usednon-aqueous solvents, it is desirable to use a device that is capable ofapplying shear, such as a bead mill, to achieve mixing under shear. Ifthe water-soluble resin used is soluble in the non-aqueous solvent used,such shearing is not necessary; however, it is preferable to achievemixing while stirring.

The average particle size of the pigment in the resulting ink ispreferably about 500 nm or less, more preferably 200 nm or less, andeven more preferably 150 nm or less. In order to reduce or eliminate theprint-through, the average particle size is preferably about 50 nm ormore. The average particle size of the pigment is a value measured usinga dynamic light-scattering particle size distribution measuring device,LB-500, available from HORIBA, Ltd.

Examples

Hereinafter, examples of the non-aqueous inkjet ink of the invention areshown.

Synthesis of Resin Solutions a to d

In a 300-ml four-necked flask, 75 g of AF-7 (naphthenic solvent,available from JX Nippon Oil & Energy Corporation) was put, and thetemperature was raised to 110° C. while introducing nitrogen gas andstirring. Then, while maintaining the temperature of 110° C., a mixtureof 16.7 g of AF-7 and 2 g of PERBUTYL 0 (t-butylperoxy-2-ethylhexanoate,available from NOF Corporation) was dripped over three hours into eachmonomer mixture according to the composition shown in Table 1. Then,while maintaining the temperature of 110° C., 0.2 g of PERBUTYL O wasadded one hour later and two hours later, respectively. The mixtureswere left to mature at 110° C. for one hour, and then were diluted with10.6 g of AF-7 to obtain colorless and transparent resin solutions a tod with a non-volatile content of 50%. The resulting resin solutions hadmass average molecular weights in the range from 20,000 to 23,000(measured according to the GPC method, converted into standardpolystyrene).

Synthesis of Resin Solution e

In a 500-ml four-necked flask, a monomer mixture according to thecomposition e shown in Table 1 was mixed. Further, 1.29 g of V-65(available from Wako Pure Chemical Industries, Ltd.) serving as thepolymerization initiator, 0.97 g of stearyl mercaptan (available fromWako Pure Chemical Industries, Ltd.) serving as the chain transferagent, and 260.2 g of AF7 (AF SOLVENT NO. 7, which is a naphthenicsolvent available from JX Nippon Oil & Energy Corporation) were added tothe mixture, and the mixture was reacted for five hours under reflux at61±3° C. to obtain a resin solution e (with a solid content of 25%).After the reaction, a trace amount of methoquinone (p-methoxyphenol)serving as the polymerization inhibitor was added. The resulting resinsolution had a mass average molecular weight of 21800 (measuredaccording to the GPC method, converted into standard polystyrene).

TABLE 1 Resin Solution a b c d e Monomer VMA Behenyl methacrylate 5032.1 Mixture (C22) (NOF Corporation) PSMA Palmityl/stearyl 50 50 50(C16/ methacrylate C18) (Kao Corporation) LMA Dodecyl methacrylate 20 3535 57.9 (C12) (Kao Corporation) EHMA 2-ethylhexyl 35 (C8) methacrylate(Mitsubishi Chemical Corporation) AAEM Acetoacetoxy 15 15ethylmethacrylate (The Nippon Synthetic Chemical Industry Co., Ltd.) GMAGlycidyl methacrylate 15 15 15 5 (NOF Corporation) DM Dimethylaminoethyl- 5 methacrylate (Wako Pure Chemical Industries, Ltd.)

Synthesis of Resin Solutions D1 and D2

In a 500-mL four-necked flask, 200 g of the resin solution a (with asolid content of 50% in the AF-7 solvent), 4.0 g of a Michael adduct(diethanolamine/2-ethylhexyl acrylate adduct), and 2.8 g ofdiethanolamine (available from Nippon Shokubai Co., Ltd.) were put, andthe temperature was raised to 110° C. while introducing nitrogen gas andstirring. Then, a reaction between the glycidyl group of the resinsolution a and diethanolamine was completed by maintaining thetemperature of 110° C. for one hour. Then, 0.2 g of dibutyl tindilaurate was added, and a mixture of 7.8 g of TAKENATE 600(1,3-bis(isocyanatomethyl) cyclohexane, available from Mitsui ChemicalsPolyurethanes, Inc.) and 72.0 g of EXEPARL HL (hexyl laurate, availablefrom Kao Corporation) was dripped over one hour. After the dripping, thetemperature was raised to 120° C. to have the mixture react for sixhours, and then the mixture was cooled to obtain a resin solution D1with a solid content of 40%.

Similarly, a resin solution D2 was prepared according to the compositionshown in Table 2. The resulting acrylic polymers had mass averagemolecular weights in the range from 22,000 to 26,000 (measured accordingto the GPC method, converted into standard polystyrene).

TABLE 2 Resin Solution D1 D2 Long-Chain Alkyl Group (Carbon Number)C16/18/12 C22/8 Main Resin Solid content 50% 200.0 0.0 Chain solution aResin Solid content 50% 0.0 200.0 solution d Side Michael adduct(diethanolamine/ 4.0 4.0 Chains 2-ethylhexyl acrylate adduct)Diethanolamine 2.8 2.8 Diisocyanate 7.8 7.8 Diluting AF-7 Naphthenicsolvent 0.0 0.0 Solvent (JX Nippon Oil & Energy Corporation) HL Hexyllaurate 72.0 72.0 (Kao Corporation) Total 286.6 286.6 Resin SolidContent 40.0 40.0 Main Chain/Side Chain Ratio 87/13 87/13

Preparation of Modified Polyethylene Imines

In a 300-ml four-necked flask, 50 g of EPOMIN SP-006 (polyethylene iminehaving a molecular weight of 600, available from Nippon Shokubai Co.,Ltd.) was put, and the temperature was raised to 60° C. while stirring.Then, 13.27 g of acrylonitrile (available from Wako Pure ChemicalIndustries, Ltd.) was dripped over about 30 minutes. After the dripping,the temperature of 60° C. was maintained for two hours to complete aMichael addition reaction to obtain a modified polyethylene imine PEI-3.Similarly, modified polyethylene imines PEI-4 to PEI-12 according to theformulations shown in Table 3 were prepared. The value of Michaeladdition equivalent shown in Table 3 means, in the case of PEI-3 where 1g of polyethylene imine (SP-006) contains 20 mmol of amino group, that avinyl compound of 20 mmol×0.5 molar equivalent=10 mmol was reacted.

TABLE 3 Polyethylene Imine Compound PEI-1 PEI-2 PEI-3 PEI-4 PEI-5 PEI-6PEI-7 PEI-8 PEI-9 PEI-10 PEI-11 PEI-12 Michael Addition Ratio (MolarEquivalent) un- un- 0.25 0.25 0.25 0.5 0.5 0.5 0.75 0.75 0.98 0.98modified modified Polyethylene SP-006 Molecular 50 50 50 50 50 50 50 5050 Imine weight 600 (available from Nippon Shokubai, Amine number: 20mmol/g, solid) SP-012 Molecular 50 50 50 weight 1200 (available fromNippon Shokubai, Amine number: 19 mmol/g, solid) Vinyl Acrylonitrile13.27 12.60 26.53 25.20 39.80 52.00 Compound Acrylate Butyl acrylate32.05 64.10 96.15 125.64 Total 50 50 63.27 82.05 62.60 76.53 114.1075.20 89.80 146.15 102.00 175.64

Preparation of Ink

4 g of the resulting resin solution a, 1.5 g of polyethylene imine(SP-012, available from Nippon Shokubai Co., Ltd.), 10 g of a pigment(carbon black, MA100, available from Mitsubishi Chemical Corporation),17.25 g of AF-7 and 17.25 g of hexyl laurate were mixed, and zirconiabeads (with a diameter of 0.5 mm) were put in the mixture to dispersethe mixture for 120 minutes using a rocking mill (available from SeiwaGiken Co., Ltd.) After the dispersion, the zirconia beads were removed,and 25 g of AF-7 and 25 g of EXEPARL HL were added to dilute themixture. Then, the diluted mixture was filtered using a 3-μm membranefilter and a 0.8-μm membrane filter in this order to remove dusts andcoarse particles to obtain an ink sample of Example 1.

Similarly, ink samples of the other examples and comparative examplesaccording to the compositions shown in Tables 4 to 8 were obtained inthe same manner as in the above-described Example 1.

An average dispersed particle size of the pigment and an ink viscosityof each of the resulting ink samples were measured. The averagedispersed particle size of the pigment was measured using a dynamiclight-scattering particle size distribution measuring device, LB-500,available from HORIBA, Ltd. The ink viscosity was a viscosity under ashear stress of 10 Pa when the shear stress was increased from 0 Pa at arate of 0.1 Pa/s at 23° C., and was measured using a stress-controlledrheometer, RS75 (with a cone angle of 10 and a diameter of 60 mm),available from Haake.

Evaluation Methods Print Density

Each of the resulting ink samples was charged in a printer, HC5500(available from Riso Kagaku Corporation), to print a solid image onplain paper (RISO printing paper (thin type), available from Riso KagakuCorporation). Then, OD values at front and back sides of the resultingprints were measured using an optical densitometer (RD920, availablefrom Macbeth), and evaluated according to the following criteria. Ahigher front side OD value indicates higher image density, and a lowerback side OD value indicates lower level of print-through.

Print Density (Front Side OD)

S: 1.20 or more;

A: 1.15 or more and less than 1.20;

B: 1.10 or more and less than 1.15;

C: 1.05 or more and less than 1.10; and

D: less than 1.05.

Print Density (Back Side OD)

S: not more than 0.20;

A: more than 0.20 and not more than 0.25;

B: more than 0.25 and not more than 0.30; and

C: more than 0.30.

Storage Stability of Ink (70° C.)

Each ink sample was put in an airtight container and was left for fourweeks in the environment of 70° C. Thereafter, a change of the inkviscosity and a change of the average dispersed particle size of thepigment of each ink sample were measured, and the results of measurementwere evaluated according to the following criteria. The viscosity andthe average dispersed particle size of the pigment of each sample afterbeing left for four weeks were measured in the manners as describedabove.

The change rate of the viscosity was calculated as follows:

[(Viscosity after four weeks×100)/(Initial value of viscosity)]−100(%).

The change rate of the average dispersed particle size of the pigmentwas calculated as follows:

[(Average dispersed particle size after four weeks×100)/(Initial valueof average dispersed particle size)]−100(%).

S: both the change rate of the viscosity and the change rate of theaverage dispersed particle size of the pigment were smaller than ±3%;

A: one of the change rate of the viscosity and the change rate of theaverage dispersed particle size of the pigment was ±3% or greater andsmaller than ±5%;

B: one of the change rate of the viscosity and the change rate of theaverage dispersed particle size of the pigment was ±5% or greater andsmaller than ±10%; and

C: one of the change rate of the viscosity and the change rate of theaverage dispersed particle size of the pigment was ±10% or greater.

Low Temperature Suitability

The resulting ink samples were left for four weeks at −5° C., and then,the ink viscosity of each ink sample at −5° C. was measured andevaluated according to the following criteria.

A: less than 50 mPa·s;

B: 50 mPa·s or more and less than 100 mPa·s; and

C: 100 mPa·s or more.

Ejection Stability

The ink samples were put in an airtight state and left for one month inthe environment of 70° C. Thereafter, using each ink sample, 100 printsof a solid image (318 dots in the main scanning direction and 3000 dotsin the sub-scanning direction) were successively printed with an inkjethead (CF1) available from Toshiba Tec Corporation, and the printedimages were visually checked and evaluated according to the followingcriteria.

A: uniform solid images were obtained;

B: uniform solid image were obtained at the early stage, and images withdefects were obtained after several tens of prints; and

C: images with defects were obtained from the early stage.

Satellites

Using each ink sample, printing on A4 paper sheets was performed with aprinter, ORPHIS-X (available from Riso Kagaku Corporation), underprinting conditions of a head gap of 3 mm, an environment temperature of15° C., a printing speed of 120 ppm and a resolution of 300 dpi*300 dpiwith 1-6 drop (6 pl/drop), and the resulting prints were evaluatedaccording to the following criteria.

A: almost no satellites were observed;

B: satellites were slightly observed; and

C: a significant level of satellites were observed.

Wetting Property to Nozzle Plate

Each inkjet ink sample was put in a 30-ml container, and one end of anozzle plate (with a length of 5 cm and a width of 5 mm), which is foruse with an inkjet printer, HC5500 (available from Riso KagakuCorporation), was picked up with tweezers to immerse the other endportion of 2 cm of the nozzle plate into the ink. In this state, thenozzle plate and the ink sample was left for a week in the environmentof 60° C., and the wetting property of each ink sample was evaluated.The nozzle plate was quickly lifted out of the ink sample, and a timetaken for the film of ink left on the nozzle plate to form ink dropletswithout wiping was measured ten times. An average value of the measuredtimes was calculated as an ink repelling time, and was evaluatedaccording to the following criteria. It should be noted that the nozzleplate used was formed by a base material of a polyimide film with thesurface thereof coated with a fluorine resin.

S: the time taken for the ink sample to form ink droplets was less than3.5 seconds;

A: the time taken for the ink sample to form ink droplets was 3.5seconds or more and less than 1 minute; and

B: the time taken for the ink sample to form ink droplets was 1 minuteor more.

The results of the above-described evaluations are shown in Tables 4 to8 together with the formulation of each ink.

TABLE 4 Example Example Example Example Example Example Example 1 2 3 45 6 7 Mass Ratio of Non-Polar 51 51 51 51 51 52 56 Solvent/Solvent inthe Ink (%) Mass Ratio of Non-Water-Soluble 1.33 1.33 1.33 10.0 0.442.67 6.67 Resin/Water-Soluble Resin Mass Ratio of Non-Water-SolubleResin/Pigment 0.2 0.2 0.2 0.2 0.2 0.4 1.0 Mass Ratio of Water-SolubleResin/Pigment 0.15 0.15 0.15 0.02 0.45 0.15 0.15 Mass Ratio of(Non-Water-Soluble 0.35 0.35 0.35 0.215 0.65 0.65 1.15 Resin +Water-Soluble Resin)/Pigment Pigment MA100 Carbon black 10.0 10.0 10.010.0 10.0 10.0 10.0 (Mitsubishi Chemical Corporation) Non- Resin Solidcontent 50% 4.0 4.0 4.0 4.0 4.0 8.0 20.0 Aqueous solution a Resin ResinSolid content 50% solution b Resin Solid content 50% solution c ResinSolid content 50% solution d Resin Solid content 25% solution e ResinSolid content 40% solution D1 Resin Solid content 40% solution D2 Water-SP-003 Molecular weight 1.5 Soluble about 300 Resin (Nippon Shokubai)(PEI) SP-012 Molecular weight 1.5 0.2 4.5 1.5 1.5 about 1200 (NipponShokubai) SP-018 Molecular weight 1.5 about 1800 (Nippon Shokubai)Diluent For AF-7 (non- Naphthenic solvent 17.25 17.25 17.25 17.93 15.7515.25 9.25 Dispersion polar solvent) (JX Nippon Oil & EnergyCorporation) HL Hexyl laurate 17.25 17.25 17.25 17.92 15.75 15.25 9.25(polar solvent) (Kao Corporation) Viscosity AF-7 (non- Naphthenicsolvent 25.0 25.0 25.0 25.0 25.0 25.0 25.0 Adjusting polar solvent) (JXNippon Oil & Solvent Energy Corporation) HL Hexyl laurate 25.0 25.0 25.025.0 25.0 25.0 25.0 (polar solvent) (Kao Corporation) Total 100.0 100.0100.0 100.0 100.0 100.0 100.0 Physical Average particle size (nm) 114124 124 118 114 106 103 Properties Viscosity (mPa · s) 9.3 9.6 9.5 9.310.3 10.5 10.9 Evaluation Print density (front side OD) S S S S A A APrint density (back side OD) S S S A A A A Storage stability S S S S A SS Low temperature suitability A A A A A A A Ejection stability A A A A AA A Satellites A A A A A A A

TABLE 5 Example Example Example Example Example Example Example Example8 9 10 11 12 13 14 15 Mass Ratio of Non-Polar 100 80 20 2 51 52 51 50Solvent/Solvent in the Ink (%) Mass Ratio of Non-Water-Soluble 1.33 1.331.33 1.33 1.33 2.67 2.67 1.33 Resin/Water-Soluble Resin Mass Ratio ofNon-Water-Soluble Resin/Pigment 0.2 0.2 0.2 0.2 0.2 0.4 0.4 0.2 MassRatio of Water-Soluble Resin/Pigment 0.15 0.15 0.15 0.15 0.15 0.15 0.150.15 Mass Ratio of (Non-Water-Soluble 0.35 0.35 0.35 0.35 0.35 0.55 0.550.35 Resin + Water-Soluble Resin)/Pigment Pigment MA100 Carbon black10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 (Mitsubishi ChemicalCorporation) Non- Resin Solid content 50% 4.0 4.0 4.0 4.0 Aqueoussolution a Resin Resin Solid content 50% 4.0 8.0 solution b Resin Solidcontent 50% solution c Resin Solid content 50% solution d Resin Solidcontent 25% solution e Resin Solid content 40% 10.0 5.0 solution D1Resin Solid content 40% solution D2 Water- SP-003 Molecular weightSoluble about 300 Resin (Nippon Shokubai) (PEI) SP-012 Molecular weight1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 about 1200 (Nippon Shokubai) SP-018Molecular weight about 1800 (Nippon Shokubai) Diluent for AF-7 (non-Naphthenic solvent 34.50 25.60 4.90 17.25 15.25 14.25 16.75 Dispersionpolar solvent) (JX Nippon Oil & Energy Corporation) HL Hexyl laurate8.90 29.60 34.50 17.25 15.25 14.25 16.75 (polar solvent) (KaoCorporation) Viscosity AF-7 (non- Naphthenic solvent 50.0 40.0 10.0 25.025.0 25.0 25.0 Adjusting polar solvent) (JX Nippon Oil & Solvent EnergyCorporation) HL Hexyl laurate 10.0 40.0 50.0 25.0 25.0 25.0 25.0 (polarsolvent) (Kao Corporation) Total 100.0 100.0 100.0 100.0 100.0 100.0100.0 100.0 Physical Average particle size (nm) 118 110 114 120 118 118111 114 Properties Viscosity (mPa · s) 8.3 8.8 9.8 10.5 9.1 9.1 9.7 9.3Evaluation Print density (front side OD) S S A A A A A A Print density(back side OD) S S A A S A S S Storage stability S S A A S S S S Lowtemperature suitability A A A A A A A A Ejection stability A A A A A A AA Satellites A A A A A A A A

TABLE 6 Example Example Example Example Example Example Example 16 17 1819 20 21 22 Pigment MA8 Carbon black 10 10 10 10 10 10 10 (MitsubishiChemical Corporation) Pigment Resin Solid content 40% 5 5 5 5 5 5 5Dispersant solution D1 Modified PEI-1 PEI PEI-2 PEI-3 PEI-4 PEI-5 PEI-61.5 PEI-7 1.5 PEI-8 1.5 PEI-9 1.5 PEI-10 1.5 PEI-11 1.5 PEI-12 1.5Diluent for AF-7 Naphthenic solvent 16.75 16.75 16.75 16.75 16.75 16.7516.75 Dispersion (JX Nippon Oil & Energy Corporation) HL Hexyl laurate16.75 16.75 16.75 16.75 16.75 16.75 16.75 (Kao Corporation) ViscosityAF-7 Naphthenic solvent 30 30 30 30 30 30 30 Adjusting (JX Nippon Oil &Solvent Energy Corporation) HL Hexyl laurate 20 20 20 20 20 20 20 (KaoCorporation) Total 100 100 100 100 100 100 100 Physical Average particlesize (nm) 124 128 136 118 114 124 130 Properties Viscosity (mPa · s) 9.69.7 9.3 9.4 9.2 9.5 9.7 Evaluation Print density (front side OD) A A A AA A A Print density (back side OD) A A A A A A A Storage stability A A AA A A A Nozzle plate wetting property evaluation S S S S S S S Lowtemperature suitability A A A A A A A Ejection stability A A A A A A ASatellites A A A A A A A

TABLE 7 Example Example Example Example Example 23 24 25 26 27 PigmentMA8 Carbon black 10 10 10 10 10 (Mitsubishi Chemical Corporation)Pigment Resin Solid content 40% 5 5 5 5 5 Dispersant solution D1Modified PEI-1 1.5 PEI PEI-2 1.5 PEI-3 1.5 PEI-4 1.5 PEI-5 1.5 PEI-6PEI-7 PEI-8 PEI-9 PEI-10 PEI-11 PEI-12 Diluent for AF-7 Naphthenicsolvent 16.75 16.75 16.75 16.75 16.75 Dispersion (JX Nippon Oil & EnergyCorporation) HL Hexyl laurate 16.75 16.75 16.75 16.75 16.75 (KaoCorporation) Viscosity AF-7 Naphthenic solvent 30 30 30 30 30 Adjusting(JX Nippon Oil & Solvent Energy Corporation) HL Hexyl laurate 20 20 2020 20 (Kao Corporation) Total 100 100 100 100 100 Physical Averageparticle size (nm) 107 112 104 108 122 Properties Viscosity (mPa · s)9.7 10.1 10.1 10.2 10.6 Evaluation Print density (front side OD) A A A AA Print density (back side OD) A A A A A Storage stability A A A A ANozzle plate wetting property evaluation B B A A A Low temperaturesuitability A A A A A Ejection stability A A A A A Satellites A A A A A

TABLE 8 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex.4 Ex. 5 Ex. 6 Ex. 7 Mass Ratio of Non-Polar 51 50 51 50 57 54 52Solvent/Solvent in the Ink (%) Mass Ratio of Non-Water-Soluble — — — —2.67 1.33 — Resin/Water-Soluble Resin Mass Ratio of Non-AqueousResin/Pigment 0.4 0.2 0.4 0.2 0.4 0.2 0.4 Mass Ratio of Water-SolubleResin/Pigment 0 0 0 0 0.15 0.15 0 Mass Ratio of (Non-Water-Soluble 0.40.2 0.4 0.2 0.4 0.2 0.4 Resin + Water-Soluble Resin)/Pigment PigmentMA100 Carbon black 10.0 10.0 10.0 10.0 10.0 10.0 10.0 (MitsubishiChemical Corporation) Non- Resin Solid content 50% 8.0 Aqueous solutiona Resin Resin Solid content 50% solution b Resin Solid content 50%solution c Resin Solid content 50% solution d Resin Solid content 25%16.0 8.0 solution e Resin Solid content 40% 10.0 5.0 solution D1 ResinSolid content 40% 10.0 5.0 solution D2 Water- SP-003 Molecular weightSoluble about 300 Resin (Nippon Shokubai) (PEI) SP-012 Molecular weight1.5 1.5 about 1200 (Nippon Shokubai) SP-018 Molecular weight about 1800(Nippon Shokubai) Diluent for AF-7 Naphthenic solvent 15.00 17.50 15.0017.50 11.25 15.25 16.00 Dispersion (non-polar (JX Nippon Oil & solvent)Energy Corporation) HL Hexyl laurate 15.00 17.50 15.00 17.50 11.25 15.2516.00 (polar solvent) (Kao Corporation) Viscosity AF-7 Naphthenicsolvent 25.0 25.0 25.0 25.0 25.0 25.0 25.0 Adjusting (non-polar (JXNippon Oil & Solvent solvent) Energy Corporation) HL Hexyl laurate 25.025.0 25.0 25.0 25.0 25.0 25.0 (polar solvent) (Kao Corporation) Total100.0 100.0 100.0 100.0 100.0 100.0 100.0 Physical Average particle size(nm) 106 222 104 Gelated 106 Gelated 244 Properties Viscosity (mPa · s)9.2 18.9 13.2 13.6 22.6 Evaluation Print density (front side OD) B B C BD Print density (back side OD) B B B B C Storage stability A C A A C Lowtemperature suitability A C C C C Ejection stability A C B B CSatellites A C C C C Comp. Comp. Comp. Comp. Comp. Comp. Ex. 8 Ex. 9 Ex.10 Ex. 11 Ex. 12 Ex. 13 Mass Ratio of Non-Polar 52 52 52 57 52 52Solvent/Solvent in the Ink (%) Mass Ratio of Non-Water-Soluble — — — —2.67 2.67 Resin/Water-Soluble Resin Mass Ratio of Non-AqueousResin/Pigment 0.4 0.4 0.4 0.4 0.4 0.4 Mass Ratio of Water-SolubleResin/Pigment 0 0 0 0 0.15 0.15 Mass Ratio of (Non-Water-Soluble 0.4 0.40.4 0.4 0.55 0.55 Resin + Water-Soluble Resin)/Pigment Pigment MA100Carbon black 10.0 10.0 10.0 10.0 10.0 10.0 (Mitsubishi ChemicalCorporation) Non- Resin Solid content 50% Aqueous solution a Resin ResinSolid content 50% 8.0 solution b Resin Solid content 50% 8.0 8.0solution c Resin Solid content 50% 8.0 8.0 solution d Resin Solidcontent 25% 16.0 solution e Resin Solid content 40% solution D1 ResinSolid content 40% solution D2 Water- SP-003 Molecular weight Solubleabout 300 Resin (Nippon Shokubai) (PEI) SP-012 Molecular weight 1.5 1.5about 1200 (Nippon Shokubai) SP-018 Molecular weight about 1800 (NipponShokubai) Diluent for AF-7 Naphthenic solvent 16.00 16.00 16.00 12.0015.25 15.25 Dispersion (non-polar (JX Nippon Oil & solvent) EnergyCorporation) HL Hexyl laurate 16.00 16.00 16.00 12.00 15.25 15.25 (polarsolvent) (Kao Corporation) Viscosity AF-7 Naphthenic solvent 25.0 25.025.0 25.0 25.0 25.0 Adjusting (non-polar (JX Nippon Oil & Solventsolvent) Energy Corporation) HL Hexyl laurate 25.0 25.0 25.0 25.0 25.025.0 (polar solvent) (Kao Corporation) Total 100.0 100.0 100.0 100.0100.0 100.0 Physical Average particle size (nm) Gelated Gelated Gelated142 203 218 Properties Viscosity (mPa · s) 14.1 33.3 38.8 EvaluationPrint density (front side OD) D C C Print density (back side OD) C B BStorage stability B C C Low temperature suitability C C C Ejectionstability C C C Satellites C C C

As shown in Tables 4 and 5, among the ink samples of Examples 1 to 15,the ink samples of Examples 1 to 13 contained an acrylic polymer formedby a copolymer of a monomer mixture including an alkyl (meth)acrylate(A) and a monomer (B), and the ink samples of Examples 14 and 15contained an acrylic polymer having a comb-shaped structure havingurethane groups as side chains to the main chain of the acrylic polymer.It can be seen from Tables 4 and 5 that all the ink samples of Examples1 to 15 had the values of the viscosity and the average dispersedparticle size of the pigment within the appropriate ranges for inkjetinks, and had excellent low temperature suitability and storagestability while achieving reduction or elimination of print-through andhigh print density. Even when the mass ratio of the non-water-solubleresin (solid content) relative to the pigment was as low as 20%, stabilepigment dispersion was achieved and penetration of the pigment wasreduced to reduce the print-through, thereby achieving high printdensity. Further, when a non-water-soluble resin and a water-solubleresin were used in combination, more improved low temperaturesuitability was obtained.

In contrast, the ink samples of Comparative Examples 1 to 4 andComparative Examples 7 to 11 shown in Table 8 did not contain awater-soluble resin. In Comparative Example 1, the pigment dispersionstability was ensured by the non-water-soluble resin, and therefore goodstorage stability, good low temperature suitability and good ejectionstability were obtained and the satellites were reduced. However, sincethe non-aqueous solvent had high affinity to the pigment, thenon-aqueous solvent dragged the pigment into the recording medium whenthe non-aqueous solvent penetrated into the recording medium, and thisresulted in the print-through and decrease of the print density. InComparative Example 2, the mass ratio of the non-water-soluble resinrelative to the pigment was half the mass ratio of the non-water-solubleresin in Comparative Example 1, and this resulted in poor pigmentdispersion stability. With respect to Comparative Examples 7 and 8,which had no side chains of urethane groups, the ink sample ofComparative Example 7 had poor pigment dispersion stability, and thepigment was not dispersed in the ink sample of Comparative Example 8. InComparative Examples 9 and 10, which did not contain thenon-water-soluble resin of the invention, the pigment was not dispersed.In Comparative Example 11, where the content of the non-water-solubleresin was doubled, the pigment was dispersed; however, the ink sample ofComparative Example 11 had poor stability and resulted in low printdensity.

In Comparative Example 3, where the carbon numbers of the alkyl groupsforming the functional groups of the non-water-soluble resin were 22 and8, the long carbon chains of the alkyl groups served to ensure thestorage stability; however, the non-water-soluble resin tended to besolidified in a low temperature environment and resulted in poor lowtemperature suitability, and the satellites were not reduced. InComparative Example 4, where the content of the non-water-soluble resinwas small, the pigment was not dispersed.

Comparative Examples 5 and 6 were the cases where the water-solubleresin was contained, but the non-water-soluble resin did not have aβ-diketone group or β-keto acid ester group. In Comparative Example 5,while the high mass ratio of the non-water-soluble resin relative to thepigment provided the pigment dispersibility even without the β-diketonegroup or β-keto acid ester group, the viscosity was increased and thisresulted in poor low temperature suitability and poor ejectionstability. Further, when compared to the ink samples of the examples ofthe invention, the pigment adsorption was insufficient because of theabsence of the β-diketone group or β-keto acid ester group, and the inksample of Comparative Example 5 failed to reduce the print-through andresulted in low print density. In Comparative Example 6, the mass ratioof the non-water-soluble was half the mass ratio of thenon-water-soluble in Comparative Example 5. In this case, the pigmentadsorption was not achieved and the pigment was not dispersed.

The ink samples of Comparative Examples 12 and 13 contained awater-soluble resin which was an acrylic polymer that did not have aβ-diketone group or β-keto acid ester group, and resulted in poor imagequality, low print density and poor stability in a low temperatureenvironment.

As can be seen from the examples of the invention, the combined use ofthe β-diketone group or β-keto acid ester group of the non-water-solubleresin and the water-soluble resin allows reducing the viscosity whileensuring the pigment dispersion stability even when the amount of thenon-water-soluble resin is small, thereby providing excellent lowtemperature suitability of the ink. Further, the water-soluble resinensures the pigment dispersion stability, and this allows reducing theamount of the non-water-soluble resin to be used, thereby reducing thepenetration of the pigment. As a result, the reduction or elimination ofthe print-through is achieved and the high print density is achieved.

On the other hand, as shown in Tables 6 and 7, the ink samples ofExamples 16 to 22 allowed ensuring the low temperature suitability andthe pigment dispersion stability and reducing the print-through at thesame time, and achieved high print density. The ink samples of Examples16 to 22 also had high ink repellency and resulted in improved wettingproperty to the nozzle plate.

In Examples 23 and 24, where the polyethylene imine was unmodified, theink repellency from the nozzle plate without wiping was insufficient. Inexamples 25 to 27, where the ratio of the modified polyethylene iminewas less than 0.3 equivalent, the ink repellency from the nozzle platewithout wiping was insufficient.

As described above, the non-aqueous pigment ink of the invention ensuresthe low temperature suitability and the pigment dispersion stability,and reduces or eliminates the print-through at the same time, therebyachieving the high print density. Therefore, the non-aqueous pigment inkof the invention is preferably usable as an inkjet ink. Further, thenon-aqueous pigment ink of the invention has low ink viscosity even in alow temperature environment, and is therefore preferably usable, inparticular, with circulation-type inkjet recording devices, whichrequire longer time and more electric power for warming-up.

What is claimed is:
 1. A non-aqueous pigment ink comprising: a pigment;a non-aqueous solvent; a non-water-soluble resin; and a water-solubleresin; wherein the non-water-soluble resin is an acrylic polymer formedby a copolymer of a monomer mixture containing at least an alkyl(meth)acrylate (A) having a C₈ to C₁₈ alkyl group and a monomer (B)having a β-diketone group or β-keto acid ester group; and the acrylicpolymer does not include a urethane group.
 2. The non-aqueous pigmentink as claimed in claim 1, wherein the water-soluble resin is apolyethylene imine having a mass average molecular weight of from 200 to2000.
 3. The non-aqueous pigment ink as claimed in claim 1, wherein acontent of the water-soluble resin is from 0.01 to 0.5 in mass ratiorelative to the pigment.
 4. The non-aqueous pigment ink as claimed inclaim 1, wherein a content of the acrylic polymer is from 0.1 to 1.0 inmass ratio relative to the pigment.
 5. The non-aqueous pigment ink asclaimed in claim 1, wherein a content of the acrylic polymer is from 0.1to 20 in mass ratio relative to the water-soluble resin.
 6. Thenon-aqueous pigment ink as claimed in claim 1, wherein the monomer (B)has a β-diketone group.
 7. The non-aqueous pigment ink as claimed inclaim 6, wherein the β-diketone group is an acetoacetyl group or apropion acetyl group.
 8. The non-aqueous pigment ink as claimed in claim1, wherein the monomer (B) has a β-keto acid ester group.
 9. Thenon-aqueous pigment ink as claimed in claim 8, wherein the β-keto acidester group is an acetoacetoxy group or a propion acetoxy group.
 10. Thenon-aqueous pigment ink as claimed in claim 1, wherein the content ofthe alkyl (meth)acrylate (A) having a C₈ to C₁₈ alkyl group in themonomer mixture is 30 mass % or more.
 11. The non-aqueous pigment ink asclaimed in claim 1, wherein the content of the monomer (B) having aβ-diketone group or β-keto acid ester group in the monomer mixture isfrom 3 mass % to 30 mass %.
 12. The non-aqueous pigment ink as claimedin claim 1, wherein the content of the alkyl (meth)acrylate (A) having aC₈ to C₁₈ alkyl group in the monomer mixture is from 50 mass % to 90mass %, and the content of the monomer (B) having a β-diketone group orβ-keto acid ester group in the monomer mixture is from 5 mass % to 20mass %.